The olecranon fossa is a deep depression on the posterior surface of the distal humerus that accommodates the olecranon process of the ulna during full elbow extension. It lies between the medial and lateral columns of the distal humerus, directly posterior to the coronoid fossa anteriorly, with only a thin bony plate separating them. The thin or sometimes perforated floor of the olecranon fossa explains why supracondylar fractures propagate easily through this region.
The posterior fat pad sign on lateral elbow radiograph, visible as a triangular lucency displaced posteriorly from the olecranon fossa, indicates an intra-articular elbow effusion pushing the posterior fat pad out of the fossa. This is an important indirect sign of occult fracture, particularly in children where posterior fat pad elevation alone (even without an anterior sail sign) is grounds for treating as a presumed occult radial head or lateral condyle fracture. Posterior elbow osteophytes from osteoarthritis project into the olecranon fossa, causing an elbow extension block.
Intra-articular effusion from fracture, haemarthrosis, or synovitis displaces the posterior fat pad from the olecranon fossa on lateral radiograph, producing a triangular lucency posterior to the distal humerus; in children this sign alone (even without a visible fracture line) indicates a presumed occult supracondylar or lateral condyle fracture requiring cast immobilisation and follow-up.
Osteophytes projecting from the olecranon tip and olecranon fossa rim in elbow osteoarthritis cause a hard mechanical block to terminal extension, managed by arthroscopic debridement of the osteophytes from the fossa through posterior portals.
